Rearrangement of Oxide Scale on Ni-11Fe-10Cu-6Al Pre-Oxidized at 950 °C during Anodic Polarization in Molten Carbonate

The effect of anodic polarization in molten Na2CO3-K2CO3 at 750 °C was investigated on the structure of oxide scale formed by pre-oxidation of Ni-11Fe-10Cu-6Al alloy at 950 °C in air. The pre-formed oxide scale evolves and rearranges under anodic polarization related to melt corrosion and non-uniformly distributed electric field. Both of pre-oxidized and as-rearranged electrodes can serve as inert anodes with oxygen evolution. Anodic polarization exhibits a negative rearrangement-destructivity effect for the pre-formed oxide scale with corrosion protection of the rearranged oxide scale decreasing. The structure rearrangement of pre-formed oxide scale is also discussed during anodic polarization in the melt.

Influence of Water Vapor and Temperature on the Oxide Scale Growth and Alpha-Case Formation in Ti-6Al-4V Alloy

The Ti-6Al-4V alloy is extensively used in aerospace, automotive and biomaterial applications. In the aerospace industry, the service temperature of Ti-6Al-4V is currently limited to 350 °C due to its insufficient oxidation resistance. Oxidation at higher temperatures causes the formation of a fast-growing oxide scale and an oxygen-enriched subsurface layer, which is known as the “alpha-case.” Additionally, the effect of water vapor on the oxidation behavior is critical. In the present study, the oxidation behavior of Ti-6Al-4V in dry air and air containing 10 vol.% H2O at 500, 600 and 700 °C for up to 500 h has been investigated. The main focus of this study is the examination of the different oxide scale morphologies along with the oxygen enrichment in the subsurface zone. It has been observed that spallation of the oxide scale is more severe in a water vapor-containing environment. In dry air, the oxide morphology shows the typical layered TiO2/Al2O3 structure after exposure at 700 °C for 300 h, while Al2O3 precipitates are present in the outermost part of the TiO2 scale when oxidized in wet air. This indicates that the solubility and diffusivity of Al3+ ions in TiO2 are influenced by water vapor. In addition, the extent of oxygen enrichment in the subsurface zone (alpha-case) as a function of temperature and time is determined by nanoindentation profiles. It was shown that in contrast to the scale formation, the alpha-case thickness is not affected by the presence of water vapor in the atmosphere.

Proving the Availability of Ni-Fe Anode for Electro-Reduction of Solid V2O3 in Molten Fluoride Salts

The availability of casting Ni-Fe alloy as inert anode for direct electro-reduction of V2O3 in molten Na3AlF6-K3AlF6-AlF3 was investigated. The electrochemical oxidation behavior of anode as well as microstructural evolutions of formed oxide scale were systematically studied. The electrochemical characterization and reaction mechanism of cathode oxide were also investigated to evaluate the influence of alloy anode on cathodic reduction process. The in situ formed three-layered oxide scale is compact and coherent, which is composed of an outermost Fe2O3+FeAl2O4 skin layer, a Fe2O3 middle layer and a FeAl2O4 inner layer. The skin layer has a continuous, smooth structure and shows electrochemical activity. The Fe2O3 layer with compact structure prevents inward diffusion of electrolyte and outward migration of metal cations. The innermost FeAl2O4 layer shows a loose structure and functions as buffer layer to improve the peeling resistance of oxide scale. With the continuous extension of polarization time, the inner FeAl2O4 layer is slowly oxidized and becomes thinner, simultaneously, the dense Fe2O3 layer becomes thicker. Ultimately, metal vanadium product with fine rod-like particles can be obtained and the oxygen content in the metal vanadium is below 0.3 mass% within electrolyzing time of 2 h. The corresponding current efficiency is around 63%.

Examining oxidation in β-NiAl and β-NiAl+Hf alloys by stochastic cellular automata simulations

We present results from a stochastic cellular automata (CA) model developed and employed for examining the oxidation kinetics of NiAl and NiAl+Hf alloys. The rules of the CA model are grounded in diffusion probabilities and basic principles of alloy oxidation. Using this approach, we can model the oxide scale thickness and morphology, specific mass change and oxidation kinetics as well as an approximate estimate of the stress and strains in the oxide scale. Furthermore, we also incorporate Hf in the grain boundaries and observe the “reactive element effect”, where doping with Hf results in a drastic reduction in the oxidation kinetics concomitant with the formation of thin, planar oxide scales. Interestingly, although we find that grain boundaries result in rapid oxidation of the undoped NiAl, they result in a slower-growing oxide and a planar oxide/metal interface when doped with Hf.